Nonlinear observer and sliding mode control design for dynamic positioning of a surface vessel

In this paper, dynamic positioning of large surface vessel in harbor area using multiple autonomous tugboats is investigated. There are two objectives in this paper. First, a nonlinear observer is applied to estimate low frequency position and velocity of ship from noisy measurements. Furthermore, the nonlinear observer is also used to estimate the bias states of slow frequency disturbances due to wind, wave and current and filter out the wave frequency components. Second, sliding mode controller for slow speed trajectory tracking is proposed. The resulting sliding mode control law guarantees desirable performance of ship considered in environmental disturbances. The proposed approach has been simulated with a supply vessel model and found to work well.